1. Field of the Invention
The present invention relates to the removal of oxides of nitrogen (NO.sub.x) from streams containing same, and more particularly but not by way of limitation, to an improved process for removing oxides of nitrogen (NO.sub.x) from chemical waste streams.
2. Brief Statement of the Prior Art
Oxides of nitrogen are one of the principal contaminants emitted by combustion processes. In every combustion process, the high temperatures at the burner result in the fixation of some oxides of nitrogen. These compounds are found in stack gases mainly as nitric oxide (NO) with lesser amounts of nitrogen dioxide (NO.sub.2) and only traces of other oxides. Since nitric oxide (NO) continues to oxidize to nitrogen dioxide (NO.sub.2) in the air at ordinary temperatures, there is no way to predict with accuracy the amounts of each separately in vented gases at a given time. Thus, the total amount of nitric oxide (NO) plus nitrogen dioxide (NO.sub.2) in a sample is determined and referred to as "oxides of nitrogen" (NO.sub.x).
Oxides of nitrogen emmissions from stack gases, through atmospheric reactions, produce "smog" that stings eyes and causes acid rains. For these reasons, the content of oxides of nitrogen present in gases vented to the atmosphere is severely limited by various state and federal agencies.
There have been considerable efforts in the art to find ways to remove the oxides of nitrogen from waste gases so that the waste gases may be discharged to the atmosphere without harm to the environment. One such prior art method utilizes an adsorption medium to adsorb the oxides of nitrogen. However, this method of removing oxides of nitrogen results in the formation of either an acidic liquid or other nitrogen containing noxious liquid streams which must be treated further before they can be safely discharged into the environment.
Another prior art process for reducing the content of oxides of nitrogen (NO.sub.x) in industrial waste gases employs the concept of reducing the oxides of nitrogen in the presence of an excess of a hydrocarbon at elevated temperatures. Such a process effectively reduces the amount of oxides of nitrogen present in the waste gases to a level that is satisfactory for release into the atmosphere. However, by-products, such as carbon monoxide, hydrogen, and carbon, are produced in such quantities that the release of the waste gases containing these by-products will cause harm to the environment unless additional steps are taken to treat the waste gases.
One process employed to reduce the amount of oxides of nitrogen (NO.sub.x) in gaseous streams so that the gaseous streams can be vented into the surrounding atmosphere without contamination of the environment, including smoke or particulate matter, is disclosed in U.S. Pat. No. 3,873,671 issued Mar. 25, 1975 and entitled "Process for Disposal of Oxides of Nitrogen". This patent teaches the burning of a hydrocarbon fuel with less than a stiochiometric amount of oxygen, the oxygen being supplied by air, by the oxides of nitrogen (NO.sub.x) present in the stream to be treated, or by a stream of air containing oxides of nitrogen. The combustion products of the hydrocarbon fuel are then mixed with gases containing oxides of nitrogen (NO.sub.x) in a ratio which provides an excess of oxidizable material under conditions that will enable a portion of the combustible products to be oxidized by oxygen made available from the decomposition of the oxides of nitrogen (NO.sub.x), thus reducing such oxides of nitrogen. This combined combustion mixture is thereafter cooled to a temperature in the range of about 2000.degree. F. to about 1200.degree. F. with a cooling fluid substantially free of oxygen. To prevent venting the excess combustibles into the atmosphere, the cooled mixture of nitrogen, combustion products and other oxidizable materials is thereafter mixed in a second zone with sufficient oxygen to convert substantially all of the oxidizable combustion products remaining to carbon dioxide and water without the reformation of oxides of nitrogen (NO.sub.x).
The process set forth in the U.S. Pat. No. 3,873,671 possesses several inherent limitations. For example, the process is limited in that the reduction of the oxides of nitrogen (NO.sub.x) occurs at a temperature below 2000.degree. F. and above 1200.degree. F. At the specified temperature range, heavy hydrocarbon fuels, such as fuel oils heavier than #2 fuel oil, unsaturated hydrocarbons, and long chain length hydrocarbons, will not be oxidized and the decomposition of oxides of nitrogen (NO.sub.x) is not favored. Further, if the combustible gases are cooled below 1400.degree. F. the oxidation of the combustibles may not be achieved because the combustible components are greatly diluted by nitrogen, carbon dioxide and water vapor. In addition, if the combustible components are not oxidized, due to the aforementioned dilution, carbon, carbon monoxide and hydrogen will be present in the vent gases. Also, if the gases are recycled to quench the combustion products, a combustible mixture will be introduced into the combustion products, i.e. the effluent from the reduction zone, at a temperature which will allow oxidation to occur. This has resulted in the deflagration of the recycle stream and burning in the recycle stream and vent gases in the stack. Such occurrences would be undesirable, as such are unsafe, since they can damage the equipment and create personnel hazard.
Numerous other processes have heretofore been disclosed for removing oxides of nitrogen (NO.sub.x) from waste gas streams so that such gas streams can be safely vented into the atmosphere. For example, some processes have employed catalysts in combination with reducing gases to selectively remove oxides of nitrogen (NO.sub.x) from gaseous streams. Other processes have employed ammonia, ammonium formate, ammonium oxalate, ammonium carbonate and the like for selectively reducing the oxides of nitrogen (NO.sub.x) in gaseous streams.
While there have been considerable efforts in the art to find effective ways to remove oxides of nitrogen (NO.sub.x) from waste gases so that the gases can be discharged into the atmosphere without harm to the environment, new and improved processes are constantly being sought which eliminate the deficiencies of the prior art processes, are safe in operation, and meet the increasingly stringent requirements being placed on the content of oxides of nitrogen (NO.sub.x) present in vented gases by federal and state agencies.